1. Field of the Invention
The present invention relates to a semiconductor radiation detection apparatus that detects radiation such as X-ray and gamma ray.
2. Description of Related Art
The semiconductor detector detects X-ray or gamma ray utilizing creation of a charge (i.e. an electron-hole pair) in a semiconductor crystal by an interaction (such as the photoelectric effect, Compton scattering, or electron pair creation) between the X-ray or gamma ray with the semiconductor crystal. The semiconductor detector has electrodes provided on two opposite sides of a semiconductor crystal and applies a bias voltage between the electrodes to thereby pickup a charge created in the crystal as signals from the respective electrodes, based on which the position at which an interaction occurs is detected.
There are various types of specific arrangements of the electrodes in semiconductor detectors. For example, there has been known a cross strip type detector that has a plurality of strip-like electrodes arranged in directions perpendicular to each other on both sides of a plate-like crystal. There also is a pixel type detector in which grid-like electrodes are provided on one side of a plate-like crystal and a single electrode is provided on the other side. Furthermore, there is a coaxial type detector using a cylindrical semiconductor having a hollow portion provided at the center thereof, in which electrodes are provided on the outer circumferential surface and the inner circumferential surface.
In any type of semiconductor detector, signals are obtained from multiple electrodes when an interaction between radiation and the semiconductor crystal occurs, and the signal obtained from the electrode provided at a position corresponding to the position at which the interaction occurs has the largest amplitude. Therefore, the position of the interaction can be detected by analyzing signals obtained from the electrodes.
However, in the above-described method of detecting the position of an interaction, the accuracy in detecting the interaction position is as fine as the size of the electrodes at best. Patent document 1 specified below discloses a semiconductor radiation detection apparatus that can detect the position of an interaction with an accuracy finer than the size of the electrodes by analyzing signal waveforms. Patent document 1 discloses a cross strip type detector in which the position of an interaction with respect to the depth direction is determined based on a difference in the rise time of signals between the anode and cathode closest to the interaction point. In addition, the position of the interaction in a plane parallel to the electrodes is determined based on the ratio of amplitudes of signals obtained from electrodes adjacent to the electrode closest to the interaction point.
Non-patent document 1 discloses another technology of detecting the position of an interaction. According to this technology, waveforms (reference waveforms) of signals obtained from an electrode when an interaction occurs at specific positions are obtained by experimental measurement and stored beforehand. In the actual detection, the waveform of a signal obtained upon detection is compared with the reference waveforms, and the position for which the waveform of the detected signal is most similar to the reference waveform is determined as the position of the interaction.    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-208057    Non-Patent Document 1: K. Vetter et. al, Three-dimensional position sensitivity in two-dimensionally segmented HP-Ge detectors, Nuclear Instruments and Method in Physics Research, A452, P. 223, 2000.